Preparing for the arrival of the invasive spongy moth

A current outbreak of an invasive insect is bad news for Pennsylvania forests. The Department of Conservation and Natural Resources(DCNR) has been working to prevent further damage.

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The spongy moth, formerly called the gypsy moth, is an invasive species that came to America during the Civil War when the cotton supply was low and entomologists at the time were looking for an alternative for the coveted silk moth to keep up with demand.

When a sample of the spongy moth population escaped from a Boston entomologist’s backyard, the moth spread like wildfire. It was held at the Hudson River with the use of DDT until the insecticide was banned in the early 1970s. That’s when the moth crossed over into Pennsylvania and New Jersey.

Their Impact Now

Fast forward to today, the spongy moth problem is still far from under control and poses an ecological threat to Pennsylvania forests. According to DCNR, the insect has killed millions of oak and other tree species across the state. 

The moth population goes through cycles with outbreaks occurring every five to 10 years. The current outbreak began in 2021.

The spongy moth eats the leaves of oak trees and other hardwoods such as beech, birches, maples, and elm.  

“Spongy moths themselves, they’re a defoliating insect…we say like 2 to 3 years of heavy defoliation consecutively to cause tree mortality,” said Rosa Yoo, forest health manager for DCNR.

Tree defoliated by spongy moths / Courtesy of DCNR

DCNR Suppression Program

DCNR is taking steps this summer to protect state-owned parks and forests, concentrating efforts in 19 counties in central and northeast Pennsylvania. 

In May, the agency did aerial spraying of a product called BtK. “It is a biocide. So it’s a bacteria that’s found naturally in the soil,” Yoo said. 

According to Yoo, these products pose no threat to human health and are safe for use, as they primarily target the spongy moth.

The areas selected for spraying are based on the density of egg masses found in annual surveys of state parks and forests.

The agency uses another agent called Mimic which essentially mimics the hormone that causes the insect to molt prematurely.

“So when they do that, they don’t survive,” Yoo said. According to the agency’s 2023 program overview, Mimic is used on the most difficult-to-control populations.

Another successful population control is a naturally-occurring fungus called Entomophaga maimaiga, which has been deliberately introduced to affected forests to help control spongy moths. The fungus embeds its spores in tree bark, ultimately killing the spongy moth caterpillar before it reaches maturity. 

“You really need wet springs for that fungus to be effective,” said Yoo. ” When you have those wet springs, it can really knock back the populations without us having to necessarily use the pesticides.” DCNR is not currently using this type of treatment.

More than 290,000 acres in 13 state forests and 18 state parks were treated this year by DCNR. In addition, 109,000 acres were treated on state game lands by the Pennsylvania Game Commission in a cooperative effort. 

Aerial surveys will take place in June and July to assess the defoliation in forests. “We kind of can see where the damage is done, then the field crew will go out and kind of ground truth to make sure that defoliation is attributed to spongy moth,” Yoo said.

After that, aerial surveys in August through October will count newly-laid egg masses to prepare for next year’s efforts.

Aircraft used for spraying / Courtesy of DCNR

What You Can Do

As for homeowners looking to protect their own hardwoods this summer into fall, the DCNR recommends removing egg masses before they hatch by scraping them into a sealed container or bag.

Also, property owners should get rid of unnecessary yard objects where egg masses can be hidden, such as piles of old wood, dead branches, and building materials. Egg masses should be scraped into a sealed container or bag for disposal.

Another control tactic is wrapping burlap around the trunks of trees where spongy moth larvae can hide during the day. They can then be scraped into a can of soapy water, killing the larvae.